Display Device with Varifocal Optical Assembly

ABSTRACT

A varifocal optical assembly includes a plurality of optical elements configured to transmit light in successive optical stages. Each respective optical stage of the successive optical stages includes at least one respective optical element of the plurality of optical elements and is configurable to be in one of a first state and a second state. The respective optical stage in the first state has a first respective optical power for light of a first polarization and a second respective optical power for light of a second polarization. The respective optical stage in the second state has a third optical power for light of the first polarization and a fourth optical power for light of the second polarization. The optical power of varifocal optical assembly is adjustable by changing the states of one or more of the successive optical stages.

RELATED APPLICATIONS

This application claims the benefit of, and priority to, U.S.Provisional Patent Application Ser. No. 62/643,691, filed Mar. 15, 2018and U.S. Provisional Patent Application Ser. No. 62/772,598, filed Nov.11, 2018. Both of these applications are incorporated by referenceherein in their entireties.

TECHNICAL FIELD

This relates generally to display devices, and more specifically tohead-mounted display devices.

BACKGROUND

Head-mounted display devices (also called herein head-mounted displays)are gaining popularity as means for providing visual information to auser. For example, head-mounted displays are used for virtual realityand augmented reality operations. A head-mounted display often includesan electronic image source and an optical assembly.

When viewing objects at different distances, the fixation point of theeyes (vergence) and the focal distance (accommodation) are normallycoupled. Accommodation is driven by retinal blur and is associated withthe distance at which the eye focuses. Vergence is driven by binocularimage disparity and is related to the fixation point of the eyes of auser. When displaying three-dimensional images in a near-eye display ora head-mounted display, the focal distance is typically fixed by theconfiguration of the image source and the optical assembly. Thus, whenobjects are simulated in three-dimensions as being at various distancesfrom the user, the fixation point of the eyes (vergence) will adjust toview the displayed object, yet the focal distance (accommodation)remains fixed, leading to decoupling of vergence and accommodation, alsoknown as vergence-accommodation conflict.

SUMMARY

In accordance with some embodiments, an optical assembly includes aplurality of optical elements configured to transmit light in successiveoptical stages. Each respective optical stage of the successive opticalstages includes at least one respective optical element of the pluralityof optical elements and configurable to be in any of a plurality ofstates including a first state and a second state. In the first state,the respective optical stage has a first respective optical power forlight of a first polarization and a second respective optical power,different from the first respective optical power, for light of a secondpolarization that is orthogonal to the first polarization. In the secondstate, the respective optical stage has a third optical power for lightof the first polarization and a fourth optical power for light of thesecond polarization. An overall optical power of the optical assembly isvariable by configuring one or more of the successive optical stages.

In some embodiments, one or more optical stages of the successiveoptical stages includes an optical element of a first type and anoptical element of a second type. The optical element of the first typeis configurable to be in an “off” state or an “on” state. In the “off”state, the optical element of the first type converts light of the firstor second polarization into light of the second or first polarization,respectively. In the “on” state, the optical element of the first typetransmits incident light without changing its polarization. The opticalelement of the second type is configured to receive light transmittedthrough the optical element of the first type and has an optical powerthat is dependent on whether the light transmitted through the opticalelement of the first type has the first polarization or the secondpolarization.

In some embodiments, one or more optical stages of the successiveoptical stages include an active optical element. The active opticalelement is configurable to be in an “off” state or an “on” state. In the“off” state, the active optical element has an optical power that isdependent on whether light incident on the active optical element hasthe first polarization or the second polarization. In the “on” state,the active optical element transmits the incident light without changingits polarization or direction.

In accordance with some embodiments, a display device includes a displayconfigured to emit image light and an optical assembly configured totransmit the image light emitted from the display. The optical assemblyincludes a plurality of optical elements configured to transmit light insuccessive optical stages. Each respective optical stage of thesuccessive optical stages includes at least one respective opticalelement of the plurality of optical elements and configurable to be inany of a plurality of states including a first state and a second state.In the first state, the respective optical stage has a first respectiveoptical power for light of a first polarization and a second respectiveoptical power, different from the first respective optical power, forlight of a second polarization that is orthogonal to the firstpolarization. In the second state, the respective optical stage has athird optical power for light of the first polarization and a fourthoptical power for light of the second polarization. An overall opticalpower of the optical assembly is variable by configuring one or more ofthe successive optical stages.

In accordance with some embodiments, a method includes transmittinglight through an optical stack having a plurality of stages andadjusting a focal length of the optical stack by changing respectivestates of one or more optical stages of the plurality of optical stages.Each stage of the plurality of optical stages is configurable to be inany of a plurality of states including a first state and a second state.In the first state, the respective optical stage has a first respectiveoptical power for light of a first polarization and a second respectiveoptical power, different from the first respective optical power, forlight of a second polarization that is orthogonal to the firstpolarization. In the second state, the respective optical stage has athird optical power for light of the first polarization and a fourthoptical power for light of the second polarization.

Thus, the disclosed embodiments provide display devices with adjustableoptical power to decrease eye fatigue and improve user comfort andsatisfaction with such devices.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various described embodiments,reference should be made to the Description of Embodiments below, inconjunction with the following drawings in which like reference numeralsrefer to corresponding parts throughout the figures. The figures are notdrawn to scale unless indicated otherwise.

FIG. 1 is a perspective view of a display device in accordance with someembodiments.

FIG. 2 is a block diagram of a system including a display device inaccordance with some embodiments.

FIG. 3A is an isometric view of a display device in accordance with someembodiments.

FIGS. 3B-3C illustrate a varifocal optical assembly in a display devicein accordance with some embodiments.

FIG. 4A illustrates a varifocal optical assembly in accordance with someembodiments.

FIG. 4B illustrates a varifocal optical assembly in accordance with someembodiments.

FIGS. 5A-5N illustrate optical properties of optical elements in avarifocal optical assembly in accordance with some embodiments.

FIGS. 6A-6D are schematic diagrams illustrating a Pancharatnam-Berryphase lens in accordance with some embodiments.

FIGS. 6E-6H are schematic diagrams illustrating a polarization sensitivehologram lens in accordance with some embodiments.

FIG. 7A illustrate optical paths of light through a varifocal opticalassembly in accordance with some embodiments.

FIGS. 7B-7C show examples of different configurations of a varifocaloptical assembly in accordance with some embodiments.

FIGS. 8A-8B illustrate a display device that includes a varifocaloptical assembly in accordance with some embodiments.

FIG. 9 illustrates a method of adjusting the focal length of lighttransmitted through a varifocal optical assembly in accordance with someembodiments.

DETAILED DESCRIPTION

The disclosed embodiments provide a varifocal optical assembly and adisplay device (e.g., a head-mounted display device) including thevarifocal optical assembly. The varifocal optical assembly includesmultiple adjustable stages that allow for the varifocal optical assemblyto have adjustable optical power such that a perceived distance of adisplayed image of an object is adjustable to match the vergence of theuser's eyes. Thus, the disclosed embodiments can be used to reduce thevergence-accommodation conflict that a user may experience while usingthe display device, thereby increasing the user's overall comfort andenjoyment while using the display device.

Reference will now be made to embodiments, examples of which areillustrated in the accompanying drawings. In the following description,numerous specific details are set forth in order to provide anunderstanding of the various described embodiments. However, it will beapparent to one of ordinary skill in the art that the various describedembodiments may be practiced without these specific details. In otherinstances, well-known methods, procedures, components, circuits, andnetworks have not been described in detail so as not to unnecessarilyobscure aspects of the embodiments.

It will also be understood that, although the terms first, second, etc.are, in some instances, used herein to describe various elements, theseelements should not be limited by these terms. These terms are used onlyto distinguish one element from another. For example, a first lightprojector could be termed a second light projector, and, similarly, asecond light projector could be termed a first light projector, withoutdeparting from the scope of the various described embodiments. The firstlight projector and the second light projector are both lightprojectors, but they are not the same light projector.

The terminology used in the description of the various describedembodiments herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used in thedescription of the various described embodiments and the appendedclaims, the singular forms “a,” “an,” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. The term “exemplary” is used herein in the senseof “serving as an example, instance, or illustration” and not in thesense of “representing the best of its kind.”

FIG. 1 illustrates display device 100 in accordance with someembodiments. In some embodiments, display device 100 is configured to beworn on a head of a user (e.g., by having the form of spectacles oreyeglasses, as shown in FIG. 1) or to be included as part of a helmetthat is to be worn by the user. When display device 100 is configured tobe worn on a head of a user or to be included as part of a helmet,display device 100 is called a head-mounted display. Alternatively,display device 100 is configured for placement in proximity of an eye oreyes of the user at a fixed location, without being head-mounted (e.g.,display device 100 is mounted in a vehicle, such as a car or anairplane, for placement in front of an eye or eyes of the user). Asshown in FIG. 1, display device 100 includes display 110. Display 110 isconfigured for presenting visual contents (e.g., augmented realitycontents, virtual reality contents, mixed reality contents, or anycombination thereof) to a user.

In some embodiments, display device 100 includes one or more componentsdescribed herein with respect to FIG. 2. In some embodiments, displaydevice 100 includes additional components not shown in FIG. 2.

FIG. 2 is a block diagram of system 200 in accordance with someembodiments. The system 200 shown in FIG. 2 includes display device 205(which corresponds to display device 100 shown in FIG. 1), imagingdevice 235, and input interface 240 that are each coupled to console210. While FIG. 2 shows an example of system 200 including one displaydevice 205, imaging device 235, and input interface 240, in otherembodiments, any number of these components may be included in system200. For example, there may be multiple display devices 205 each havingassociated input interface 240 and being monitored by one or moreimaging devices 235, with each display device 205, input interface 240,and imaging devices 235 communicating with console 210. In alternativeconfigurations, different and/or additional components may be includedin system 200. For example, in some embodiments, console 210 isconnected via a network (e.g., the Internet) to system 200 or isself-contained as part of display device 205 (e.g., physically locatedinside display device 205). In some embodiments, display device 205 isused to create mixed reality by adding in a view of the realsurroundings. Thus, display device 205 and system 200 described here candeliver augmented reality, virtual reality, and mixed reality.

In some embodiments, as shown in FIG. 1, display device 205 is ahead-mounted display that presents media to a user. Examples of mediapresented by display device 205 include one or more images, video,audio, or some combination thereof. In some embodiments, audio ispresented via an external device (e.g., speakers and/or headphones) thatreceives audio information from display device 205, console 210, orboth, and presents audio data based on the audio information. In someembodiments, display device 205 immerses a user in an augmentedenvironment.

In some embodiments, display device 205 also acts as an augmentedreality (AR) headset. In these embodiments, display device 205 augmentsviews of a physical, real-world environment with computer-generatedelements (e.g., images, video, sound, etc.). Moreover, in someembodiments, display device 205 is able to cycle between different typesof operation. Thus, display device 205 operate as a virtual reality (VR)device, an augmented reality (AR) device, as glasses or some combinationthereof (e.g., glasses with no optical correction, glasses opticallycorrected for the user, sunglasses, or some combination thereof) basedon instructions from application engine 255.

Display device 205 includes electronic display 215, one or moreprocessors 216, eye tracking module 217, one or more locators 220, oneor more position sensors 225, one or more position cameras 222, memory228, controller 231, optics 260 or a subset or superset thereof (e.g.,display device 205 with electronic display 215, one or more processors216, and memory 228, without any other listed components). Someembodiments of display device 205 have different modules than thosedescribed here. Similarly, the functions can be distributed among themodules in a different manner than is described here.

One or more processors 216 (e.g., processing units or cores) executeinstructions stored in memory 228. Memory 228 includes high-speed randomaccess memory, such as DRAM, SRAM, DDR RAM or other random access solidstate memory devices; and may include non-volatile memory, such as oneor more magnetic disk storage devices, optical disk storage devices,flash memory devices, or other non-volatile solid state storage devices.Memory 228, or alternately the non-volatile memory device(s) withinmemory 228, includes a non-transitory computer readable storage medium.In some embodiments, memory 228 or the computer readable storage mediumof memory 228 stores programs, modules and data structures, and/orinstructions for displaying one or more images on electronic display215.

Electronic display 215 displays images to the user in accordance withdata received from console 210 and/or processor(s) 216. In variousembodiments, electronic display 215 may comprise a single adjustabledisplay element or multiple adjustable display elements (e.g., a displayfor each eye of a user).

In some embodiments, the display element includes one or more lightemission devices and a corresponding array of spatial light modulators.A spatial light modulator is an array of electro-optic pixels,opto-electronic pixels, some other array of devices that dynamicallyadjust the amount of light transmitted by each device, or somecombination thereof. These pixels are placed behind optics 260 In someembodiments, the spatial light modulator is an array of liquid crystalbased pixels in an LCD (a Liquid Crystal Display). Examples of the lightemission devices include: an organic light emitting diode, anactive-matrix organic light-emitting diode, a light emitting diode, sometype of device capable of being placed in a flexible display, or somecombination thereof. The light emission devices include devices that arecapable of generating visible light (e.g., red, green, blue, etc.) usedfor image generation. The spatial light modulator is configured toselectively attenuate individual light emission devices, groups of lightemission devices, or some combination thereof. Alternatively, when thelight emission devices are configured to selectively attenuateindividual emission devices and/or groups of light emission devices, thedisplay element includes an array of such light emission devices withouta separate emission intensity array.

Optics 260 direct light from the arrays of light emission devices(optionally through the emission intensity arrays) to locations withineach eyebox and ultimately to the back of the user's retina(s). Aneyebox is a region that is occupied by an eye of a user locatedproximity to display device 205 (e.g., a user wearing display device205) for viewing images from display device 205. In some cases, theeyebox is represented as a 10 mm×10 mm square. In some embodiments,optics 260 include one or more coatings, such as anti-reflectivecoatings.

In some embodiments, the display element includes an infrared (IR)detector array that detects IR light that is retro-reflected from theretinas of a viewing user, from the surface of the corneas, lenses ofthe eyes, or some combination thereof. The IR detector array includes anIR sensor or a plurality of IR sensors that each correspond to adifferent position of a pupil of the viewing user's eye. In alternateembodiments, other eye tracking systems may also be employed.

Eye tracking module 217 determines locations of each pupil of a user'seyes. In some embodiments, eye tracking module 217 instructs electronicdisplay 215 to illuminate the eyebox with IR light (e.g., via IRemission devices in the display element).

A portion of the emitted IR light will pass through the viewing user'spupil and be retro-reflected from the retina toward the IR detectorarray, which is used for determining the location of the pupil.Alternatively, the reflection off of the surfaces of the eye is used toalso determine location of the pupil. The IR detector array scans forretro-reflection and identifies which IR emission devices are activewhen retro-reflection is detected. Eye tracking module 217 may use atracking lookup table and the identified IR emission devices todetermine the pupil locations for each eye. The tracking lookup tablemaps received signals on the IR detector array to locations(corresponding to pupil locations) in each eyebox. In some embodiments,the tracking lookup table is generated via a calibration procedure(e.g., user looks at various known reference points in an image and eyetracking module 217 maps the locations of the user's pupil while lookingat the reference points to corresponding signals received on the IRtracking array). As mentioned above, in some embodiments, system 200 mayuse other eye tracking systems than the embedded IR one describedherein.

Optional locators 220 are objects located in specific positions ondisplay device 205 relative to one another and relative to a specificreference point on display device 205. A locator 220 may be a lightemitting diode (LED), a corner cube reflector, a reflective marker, atype of light source that contrasts with an environment in which displaydevice 205 operates, or some combination thereof. In embodiments wherelocators 220 are active (i.e., an LED or other type of light emittingdevice), locators 220 may emit light in the visible band (e.g., about400 nm to 750 nm), in the infrared band (e.g., about 750 nm to 1 mm), inthe ultraviolet band (about 100 nm to 400 nm), some other portion of theelectromagnetic spectrum, or some combination thereof.

In some embodiments, locators 220 are located beneath an outer surfaceof display device 205, which is transparent to the wavelengths of lightemitted or reflected by locators 220 or is thin enough to notsubstantially attenuate the wavelengths of light emitted or reflected bylocators 220. Additionally, in some embodiments, the outer surface orother portions of display device 205 are opaque in the visible band ofwavelengths of light. Thus, locators 220 may emit light in the IR bandunder an outer surface that is transparent in the IR band but opaque inthe visible band.

Imaging device 235 generates calibration data in accordance withcalibration parameters received from console 210. Calibration dataincludes one or more images showing observed positions of locators 220that are detectable by imaging device 235. In some embodiments, imagingdevice 235 includes one or more still cameras, one or more videocameras, any other device capable of capturing images including one ormore locators 220, or some combination thereof. Additionally, imagingdevice 235 may include one or more filters (e.g., used to increasesignal to noise ratio). Imaging device 235 is configured to optionallydetect light emitted or reflected from locators 220 in a field of viewof imaging device 235. In embodiments where locators 220 include passiveelements (e.g., a retroreflector), imaging device 235 may include alight source that illuminates some or all of locators 220, whichretro-reflect the light towards the light source in imaging device 235.Second calibration data is communicated from imaging device 235 toconsole 210, and imaging device 235 receives one or more calibrationparameters from console 210 to adjust one or more imaging parameters(e.g., focal length, focus, frame rate, ISO, sensor temperature, shutterspeed, aperture, etc.).

Input interface 240 is a device that allows a user to send actionrequests to console 210. An action request is a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.Input interface 240 may include one or more input devices. Example inputdevices include: a keyboard, a mouse, a game controller, data from brainsignals, data from other parts of the human body, or any other suitabledevice for receiving action requests and communicating the receivedaction requests to console 210. An action request received by inputinterface 240 is communicated to console 210, which performs an actioncorresponding to the action request. In some embodiments, inputinterface 240 may provide haptic feedback to the user in accordance withinstructions received from console 210. For example, haptic feedback isprovided when an action request is received, or console 210 communicatesinstructions to input interface 240 causing input interface 240 togenerate haptic feedback when console 210 performs an action.

Console 210 provides media to display device 205 for presentation to theuser in accordance with information received from one or more of:imaging device 235, display device 205, and input interface 240. In theexample shown in FIG. 2, console 210 includes application store 245,tracking module 250, and application engine 255. Some embodiments ofconsole 210 have different modules than those described in conjunctionwith FIG. 2. Similarly, the functions further described herein may bedistributed among components of console 210 in a different manner thanis described here.

When application store 245 is included in console 210, application store245 stores one or more applications for execution by console 210. Anapplication is a group of instructions, that when executed by aprocessor, is used for generating content for presentation to the user.Content generated by the processor based on an application may be inresponse to inputs received from the user via movement of display device205 or input interface 240. Examples of applications include: gamingapplications, conferencing applications, video playback application, orother suitable applications.

When tracking module 250 is included in console 210, tracking module 250calibrates system 200 using one or more calibration parameters and mayadjust one or more calibration parameters to reduce error indetermination of the position of display device 205. For example,tracking module 250 adjusts the focus of imaging device 235 to obtain amore accurate position for observed locators on display device 205.Additionally, if tracking of display device 205 is lost (e.g., imagingdevice 235 loses line of sight of at least a threshold number oflocators 220), tracking module 250 re-calibrates some or all of system200.

In some embodiments, tracking module 250 tracks movements of displaydevice 205 using second calibration data from imaging device 235. Forexample, tracking module 250 determines positions of a reference pointof display device 205 using observed locators from the secondcalibration data and a model of display device 205. In some embodiments,tracking module 250 also determines positions of a reference point ofdisplay device 205 using position information from the first calibrationdata. Additionally, in some embodiments, tracking module 250 may useportions of the first calibration data, the second calibration data, orsome combination thereof, to predict a future location of display device205. Tracking module 250 provides the estimated or predicted futureposition of display device 205 to application engine 255.

Application engine 255 executes applications within system 200 andreceives position information, acceleration information, velocityinformation, predicted future positions, or some combination thereof ofdisplay device 205 from tracking module 250. Based on the receivedinformation, application engine 255 determines content to provide todisplay device 205 for presentation to the user. For example, if thereceived information indicates that the user has looked to the left,application engine 255 generates content for display device 205 thatmirrors the user's movement in an augmented environment. Additionally,application engine 255 performs an action within an applicationexecuting on console 210 in response to an action request received frominput interface 240 and provides feedback to the user that the actionwas performed. The provided feedback may be visual or audible feedbackvia display device 205 or haptic feedback via input interface 240.

FIG. 3A is an isometric view of display device 300 in accordance withsome embodiments. FIG. 3A shows some of the components of display device205, such as electronic display 205 and optics 260. In some otherembodiments, display device 300 is part of some other electronic display(e.g., a digital microscope, a head-mounted display device, etc.). Insome embodiments, display device 300 includes light emission devicearray 310 and optical assembly 330. In some embodiments, display device300 also includes an IR detector array.

Light emission device array 310 emits image light and optional IR lighttoward the viewing user. Light emission device array 310 may be, e.g.,an array of LEDs, an array of microLEDs, an array of OLEDs, or somecombination thereof. Light emission device array 310 includes lightemission devices 320 that emit light in the visible light (andoptionally includes devices that emit light in the IR).

In some embodiments, display device 300 includes an emission intensityarray configured to selectively attenuate light emitted from lightemission array 310. In some embodiments, the emission intensity array iscomposed of a plurality of liquid crystal cells or pixels, groups oflight emission devices, or some combination thereof. Each of the liquidcrystal cells is, or in some embodiments, groups of liquid crystal cellsare, addressable to have specific levels of attenuation. For example, ata given time, some of the liquid crystal cells may be set to noattenuation, while other liquid crystal cells may be set to maximumattenuation. In this manner, the emission intensity array is able tocontrol what portion of the image light emitted from light emissiondevice array 310 is passed to optical assembly 330. In some embodiments,display device 300 uses an emission intensity array to facilitateproviding image light to a location of pupil 350 of eye 340 of a user,and minimize the amount of image light provided to other areas in theeyebox.

Optical assembly 330 receives the modified image light (e.g., attenuatedlight) from emission intensity array (or directly from emission devicearray 310), and direct the modified image light to a location of pupil350.

In some embodiments, display device 300 includes one or more broadbandsources (e.g., one or more white LEDs) coupled with a plurality of colorfilters, in addition to, or instead of, light emission device array 310.

FIGS. 3B and 3C illustrate display device 302, corresponding to displaydevice 300, in accordance with some embodiments. Display device 302includes a display 360, a lens assembly 362, and a varifocal opticalassembly 364. Referring to FIG. 3B, varifocal optical assembly 364 isconfigured to have a first optical power such that objects displayed bydisplay 360 is perceived by a user's eye 340 to be at a first imageplane 366, located at a first distance 361 behind display 360. Incontrast, FIG. 3C shows varifocal optical assembly 364 configured tohave a second optical power that is different from (in this case,greater than) the first optical power. Thus, the objects displayed bydisplay 360 is perceived by a user's eye 340 to be at a second imageplane 368, located at a second distance 363 behind display 360 that isfurther than first distance 361. Although FIGS. 3B and 3C show lensassembly 362 located between display 360 and varifocal optical assembly364, varifocal optical assembly 364 may also be located between display360 and lens assembly 362.

FIG. 4A illustrates a varifocal optical assembly 400 corresponding tovarifocal optical assembly 364 in accordance with some embodiments.Optical assemblies, in general, can be used to provide focusing powerfor a display device. The disclosed embodiments utilize varifocaloptical assembly 400 to enable display devices to have adjustableoptical power. In some embodiments, varifocal optical assembly 400corresponds to optical assembly 330. In some embodiments, optics 260includes varifocal optical assembly 400.

As shown in FIG. 4A, varifocal optical assembly 400 includes a pluralityof successive optical stages 420-1, 420-2, . . . , 420-n (also referredto herein as “optical stage 420”) configured to transmit light (e.g.,light 401) at various optical powers. Except for a first optical stage420-1, each respective optical stage of the successive optical stagesreceives incident light that is output from a prior stage. For example,as shown, second optical stage 420-2 receives light 401-2 that is outputfrom first stage 420-1. In some embodiments, each respective stage ofthe successive optical stages 420 is configurable to be in any of aplurality of states including a first state and a second state. In thefirst state, the respective optical stage has a first respective opticalpower for light of a first polarization and a second respective opticalpower, different from the first respective optical power, for light of asecond polarization that is orthogonal to the first polarization. In thesecond state, the respective optical stage has a third optical power forlight of the first polarization and a fourth optical power for light ofthe second polarization. As a result, an overall optical power ofvarifocal optical assembly 400 is variable by configuring one or more ofthe successive optical stages 420.

In some embodiments, varifocal optical assembly 400 is configured tohave an overall optical power that can be at any of at least threedifferent levels of optical power for two optical stages (e.g., n=2).For example, varifocal optical assembly 400 having two stages may have avariable overall optical power that can be any of at least threedifferent values, such as −2 diopters, 0 diopters, and +2 diopters. Infurther embodiments, varifocal optical assembly 400 is configured tohave an overall optical power that can be at any of at least fourdifferent levels of optical power for two or more optical stages (e.g.,n>=2). For example, varifocal optical assembly 400 can have a variableoverall optical power that can be any of at least four different values,such as −1.5 diopters, −0.5 diopters, +0.5 diopters, and +1.5 diopterswhen, for example, varifocal optical assembly 400 includes two opticalstages, one optical stage with a switchable retarder and a PBP lenshaving 0.5 or −1.5 diopter optical power, and another optical stage witha switchable retarder and a PBP lens having 0.5 or −0.5 diopter opticalpower. In further embodiments, varifocal optical assembly 400 isconfigured to have an overall optical power that can be at any of atleast four different levels of optical power for two or more opticalstages (e.g., n≥2). For example, varifocal optical assembly 400 can havea variable overall optical power that can be any of at least fourdifferent values, such as −1.5 diopters, −0.5 diopters, +0.5 diopters,and +1.5 diopters, when, for example, varifocal optical assembly 400includes at least two stages, one optical stage (e.g., an active PBPlens) having −1, 0, or 1 diopter optical power, and another opticalstage (e.g., a switchable retarder and a PBP lens) having −0.5, or +0.5diopter optical power, or when, for example, varifocal optical assembly400 includes more than two stages, such as the examples shown in FIGS.7B and 7C. In another example, varifocal optical assembly 400 can have avariable overall optical power that can be any of at least fivedifferent values, such as −1.5 diopters, −0.5 diopters, 0, +0.5diopters, and +1 diopters, when, for example, varifocal optical assembly400 includes at least two stages that each have at least one activesecond optical element (e.g., active PBP lens). For example, one opticalstage having −1, 0, or 1 diopter optical power and another optical stagehaving −0.5, 0, or +0.5 diopter optical power. The overall optical powercan have a larger number of different levels of optical power by addingmore stages, or by including one or more stages each having an activeliquid-crystal optical phase array adjustable lens with continuouslytunable optical power within a certain range. In some embodiments,varifocal optical assembly 400 may further include one or more opticalelements 405 before the first optical stage and/or one or more opticalelements 406 after a last optical stage 420-n.

FIG. 4B illustrates an example of varifocal optical assembly 400, inwhich each of the successive optical stages 420 includes a pair ofoptical elements, at least one of which is configurable to be in eitherof two different states, according to some embodiments. Thus, as shownin FIG. 4B, varifocal optical assembly 400 may include a plurality ofoptical elements of a first type 410-1, 410-2, . . . , 410-m, and 410-n(also referred to herein as “first optical elements 410” or “opticalelements of the first type 410”) and a plurality of optical elements ofa second type 412-1, 412-2, . . . , 412-m, and 412-n (also referred toherein as “second optical elements 412” or “optical elements of thesecond type 412”).

In some embodiments, the plurality of optical elements of the first type410 and the plurality of optical elements of the second type 412 form aplurality of pairs of optical elements corresponding to an opticalstage. In such cases, a respective pair of optical elements,corresponding to an optical stage 420, includes a respective opticalelement of the first type 410 and a respective optical element of thesecond type 412. The respective optical element of the first type 410 isconfigurable via a respective controller 414 to be in a first opticalelement state or a second optical element state. In the first opticalelement state, the respective optical element of the first type 410converts light of a first or second polarization into light of a secondor first polarization, respectively. The first polarization isorthogonal to the second polarization. In the second optical elementstate, the respective optical element of the first type 410 transmitsincident light without changing its polarization. The respective opticalelement of the second type 412 has a first respective optical power forlight of a first polarization and a second respective optical power,different from the first optical power, for light of a secondpolarization that is orthogonal to the first polarization. Therespective optical element of the second type 412 is configured toreceive light transmitted through the respective optical element of thefirst type 410.

The optical stages 420 are arranged successively in the optical stacksuch that light is transmitted through the optical stack in a pluralityof successive optical stages 420. In some embodiments, the plurality ofsuccessive optical stages 420 correspond to respective ones of theplurality of optical element pairs. In some embodiments, the state of arespective optical stage 420 corresponds to the state of the opticalelement of the first type 410 associated with the respective opticalstage. As shown, the optical stack has an input side and an output side.A first optical stage 420-1, corresponding to a first optical elementpair, is located at the input side of the optical stack and a lastoptical stage 420-n, corresponding to a last optical element pair, islocated at the output side of the optical stack. Additionally, in someembodiments, the optical stack also includes one or more additionalstages (e.g., additional optical stages 420-2, . . . , 420-m), referredto hereafter collectively or individually as additional optical stages422, between the first stage and the last stage.

In some embodiments, as shown in FIG. 4B, each optical element pairincludes a first optical element 410 and a second optical element 412that is configured to receive light transmitted through the firstoptical element 410. The first optical element 410 is electricallyconnected to controller 414 (e.g., controller 414-1, 414-2, 414-m, or414-n), referred to hereafter individually and collectively as 414,which is configured to control (e.g., adjust) the state of first opticalelement 410.

The first optical element 410 is configurable to be in a first opticalelement state (e.g., an “off” state) or a second optical element state(e.g., an “on” state). In the first optical element state, first opticalelement 410 is configured to convert incident light to transmitted lighthaving different polarization from that of the incident light. In thesecond optical element state, first optical element 410 is configured totransmit incident light without changing its polarization. For example,when first optical element 410 is set to the first state, leftcircularly polarized (LCP) light incident upon first optical element 410will be transmitted as right circularly polarized (RCP) light, and viceversa. In contrast, when first optical element 410 is set to the secondstate, incident upon first optical element 410 will be transmittedwithout a change in its polarization (e.g., LCP light remains LCP andRCP light remains RCP). In some embodiments, the first optical element410 is a switchable retarder or switchable wave plate, such as aswitchable half-wave plate.

The second optical element 412 has a first respective optical power forlight of a first polarization and a second respective optical power,different from the first optical power, for light of a secondpolarization that is orthogonal to the first polarization.

In some embodiments, the second respective optical power is less thanthe first respective optical power. In some embodiments, the secondrespective optical power is zero. For example, second optical element412 may have a first optical power that is non-zero for RCP light and isconfigured convert the RCP light to LCP light while converging ordiverging (depending on the first optical power) the RCP light. Thesecond optical element is also configured to transmit LCP light withoutchanging the direction or polarization of the LCP light.

In some embodiments, the second respective optical power is about equalin magnitude to the first respective optical power but is opposite ineffect from the first respective optical power. For example, secondoptical element 412 may act as a positive lens that has an optical powerof +0.5 diopters for RCP light and may act as a negative lens that hasan optical power of −0.5 diopters for LCP light. Thus, the optical powerof the second optical element 412, and therefore the optical power ofthe corresponding optical stage, is based on the state of thecorresponding first optical element 410 and the polarization of lightincident on the respective optical stage.

In some embodiments, the second optical element 412 is a polarizationsensitive optical element. In some embodiments, the second opticalelement 412 includes one or more of a Pancharatnam-Berry phase (PBP)lens (also called a geometric phase lens), a PBP grating (also called ageometric phase grating), a polarization sensitive hologram (PSH) lens,a PSH grating, and a liquid crystal optical phase array. Detailsregarding PBP lens and PSH lens are provided below with respect to FIGS.6A-6D and FIGS. 6E-6H, respectively.

In some embodiments, the second optical element 412 includes a thin filmformed on a surface of the corresponding first optical element. Forexample, the second optical element 412 may be a coating or a thin filmthat is located/deposited on a surface of the corresponding firstoptical element 410.

In some embodiments, a respective second optical element 412 has arespective optical power. In some embodiments, a magnitude of theoptical power of any second optical element 412 is no greater than 2.0diopters (e.g., the optical power is no stronger than −2 diopters or +2diopters). In some embodiments, a second optical element 412 of anoptical stage has an optical power that is different from another secondoptical element 412 of another optical stage. In some embodiments, thesecond optical element 412 -1 of the first optical stage 420-1 has afirst optical power and the last optical element 412-n of the lastoptical stage 420-n has a second optical power that is different fromthe first optical power. In some embodiments, the second optical poweris larger in magnitude than the first optical power.

In some embodiments, as shown in FIG. 4B, one or more of the successiveoptical stages 420-1, 420-2, . . . , 420-n each includes an activesecond optical element 412 that is configurable via a respectivecontroller 416 (e.g., controllers 416-1, 416-2, 416-m, 416-n) to be inany of a third optical element state (e.g., an “off” state) and a fourthoptical element state (e.g., an “on” state). In the third opticalelement state, the active second optical element 412 is configured tohave the first respective optical power for incident light having thefirst polarization and the second respective optical power for incidentlight having the second polarization. In the fourth optical elementstate, the active second optical element 412 is configured to have zerooptical power and is configured to transmit the incident light withoutexerting optical power regardless of polarization of the incident light.In some embodiments, such as when the second optical element 412 is anactive PSH optical element, the second respective optical power is zero.In some embodiments, such as when the second optical element is anactive PBP optical element, the second respective optical power is equalin magnitude and opposite in effect to the first respective opticalpower. As a result, a particular optical stage (e.g. stage 420-2)including a first optical element (e.g., optical element 410-2) and anactive second optical element (e.g., optical element 412-2) can havemore than two different states depending on the states of the firstoptical element and the active second optical element in the particularstage.

In some embodiments, one or more optical stages of the successiveoptical stages 420 includes only one of a first optical element 410 andan active second optical element 412. For example, an optical stage ofthe successive optical stages 420 may include active second opticalelement without including first optical element 410.

In general, the optical stack is configured to receive light at theinput end, transmit the light through the optical stack (e.g., throughthe first optical stage 420-1, the one or more additional stages 422,and the last optical stage 420-n), and output the light at the outputend of the optical stack such that the divergence of the light ischanged.

Thus, the overall optical power of varifocal optical assembly 400 isadjustable by adjusting or changing the respective states of the opticalstages 420.

In some embodiments, when an optical stage 420 includes an opticalelement pair, the overall optical power of varifocal optical assembly400 can be adjusted by adjusting or changing the respective states ofthe first optical elements 410 in the plurality of optical stages 420.The optical power of the optical stack can be changed by switching thestate of the first optical elements 410 in any optical stage, therebychanging the optical power of the optical stage. The optical powers ofthe successive optical stages 420 in combination determine the resultanttotal optical power of the optical stack.

FIGS. 5A-5D illustrate optical properties of an optical stage (e.g.,optical stage 420) corresponding to an optical element pair 500 of avarifocal optical assembly (e.g., varifocal optical assembly 400) inaccordance with some embodiments. Optical element pair 500 includes aswitchable optical element 510, corresponding to first optical element410, that is electrically coupled to controller 514, corresponding tocontroller 414. In some embodiments, switchable optical element 510 is aswitchable half-wave plate.

Optical element pair 500 also includes optical element 512,corresponding to second optical element 412 in accordance with someembodiments. Optical element 512 has a positive optical power for LCPlight and a negative optical power for RCP light. In other words,optical element 512 is configured to act as a positive lens (e.g.,converging lens) when LCP light is incident on optical element 512. WhenRCP light is incident on optical element 512, optical element 512 isconfigured to act as a negative lens (e.g., diverging lens). In someembodiments, optical element 512 is a PBP lens, described below withrespect to FIGS. 6A-6D.

FIGS. 5A-5D illustrate optical properties of an optical stage thatincludes an optical element pair 500.

Referring to FIG. 5A, switchable optical element 510, in the firstoptical element state (corresponding to a first state of the opticalstage), receives incident RCP light 520-A and converts RCP light 520-Ainto LCP light 520-B. Optical element 512 receives LCP light 520-Boutput from switchable optical element 510, and converts LCP light 520-Binto RCP light 520-C while converging it. In some embodiments, when LCPlight 520-B is substantially collimated, RCP light 520-C is focused byoptical element 512 to a focal point located a distance 530 away fromoptical element 512.

Referring to FIG. 5B, switchable optical element 510, in the firstoptical element state, receives incident LCP light 524-A converts LCPlight 524-A into RCP light 524-B. Optical element 512 receives RCP light524-B output from switchable optical element 510 and converts RCP light524-B as LCP light 524-C while diverging it. In some embodiments, whenRCP light 524-B is substantially collimated, LCP light 524-C is divergedby optical element 512 such that a virtual focal point of the diverginglight would be located at distance 530 away from optical element 512.

Referring to FIG. 5C switchable optical element 510, in the secondoptical element state (corresponding to a second state of the opticalstage), receives incident RCP light 520-A and transmits RCP light 520-Aas RCP light 522-B. Optical element 512 receives RCP light 522-B outputfrom switchable optical element 510 and converts RCP light 522-B intoLCP light 522-C while diverging it. In some embodiments, when RCP light522-B is substantially collimated, LCP light 522-C is diverged byoptical element 512 such that a virtual focal point of the diverginglight would be located at distance 530 away from optical element 512.

Referring to FIG. 5D, switchable optical element 510, in the secondstate, receives incident LCP light 524-A and transmits LCP light 524-Aas LCP light 526-B. Optical element 512 receives LCP light 526-B outputfrom switchable optical element 510 and converts LCP light 526-B as RCPlight 526-C while converging it. In some embodiments, when LCP light526-B is substantially collimated, RCP light 526-C is focused by opticalelement 512 to a focal point located a distance 530 away from opticalelement 512.

FIGS. 5E-5H illustrate optical properties of an optical stage includingan optical element pair 502 (e.g., optical element pair 420) of avarifocal optical assembly (e.g., varifocal optical assembly 400) inaccordance with some embodiments. Optical element pair 502 is similar tooptical element pair 500 except that optical element pair 502 includesoptical element 513 instead of optical element 512. Optical element 513is configured to have a positive optical power for RCP light and anegative optical power for LCP light. In other words, optical element513 is configured to act as a positive lens (e.g., converging lens) whenRCP light is incident on optical element 513 and to act as a negativelens (e.g., diverging lens) when LCP light is incident on opticalelement 513.

Referring to FIG. 5E, switchable optical element 510, in the firststate, receives incident RCP light 525-A and converts RCP light 525-Ainto LCP light 525-B. Optical element 513 receives LCP light 525-Boutput from switchable optical element 510 and converts LCP light 525-Binto RCP light 525-C while diverging it. In some embodiments, when LCPlight 525-B is substantially collimated, RCP light 525-C is diverged byoptical element 513 such that a virtual focal point of the diverginglight would be located at distance 532 away from optical element 513.

Referring to FIG. 5F, switchable optical element 510, in the firstoptical element state, receives incident LCP light 521-A and convertsLCP light 521-A into RCP light 521-B. Optical element 513 receives RCPlight 521-B output from switchable optical element 510 and converts RCPlight 521-B as LCP light 521-C while converging it. In some embodiments,when RCP light 521-B is substantially collimated, LCP light 521-C isfocused by optical element 513 to a focal point located a distance 532away from optical element 513.

Referring to FIG. 5G, switchable optical element 510, in the secondstate, receives incident RCP light 525-A and transmits light 525-A asRCP light 527-B. Optical element 513 receives RCP light 527-B outputfrom switchable optical element 510 and converts RCP light 527-B intoLCP light 527-C while converging it. In some embodiments, when light RCPlight 527-B is substantially collimated, LCP light 527-C is focused byoptical element 513 to a focal point located a distance 532 away fromoptical element 513.

Referring to FIG. 5H, switchable optical element 510, in the secondstate, receives incident LCP light 521-A and transmits LCP light 521-Aas LCP light 523-B. Optical element 513 receives LCP light 523-B outputfrom switchable optical element 510 and converts LCP light 523-B as RCPlight 523-C while diverging it. In some embodiments, when LCP light523-B is substantially collimated, RCP light 523-C is diverged byoptical element 513 such that a virtual focal point of the diverginglight would be located at distance 532 away from optical element 513.

In some embodiments, second optical elements 512 and/or 513 can beactive optical elements configurable via controller 516 to be in a thirdoptical element state (e.g., “off” state), as shown in FIGS. 5A-5H, or afourth optical element state (e.g., “on” state), as shown in FIG. 5I.

In some embodiments, optical element 512, 513 is a PBP lens or an activePBP lens. In some embodiments, a polarization sensitive hologram opticalelement (e.g., PSH lens or active PSH lens) may be included in anoptical stage (e.g., optical element pair 500 or 502) in place ofoptical element 512 or optical element 513.

FIGS. 5J-5K illustrate optical properties of polarization sensitivehologram optical elements 515, which can be used in place of opticalelement 512 or optical element 513 in optical element pair 500 or 502,respectively, in accordance with some embodiments.

As shown in FIGS. 5J and 5K, optical element 515 has a non-zero opticalpower for RCP light and zero optical power for LCP light. In otherwords, optical element 515 is configured to act as a lens when RCP lightis incident on optical element 515 and convert the RCP light into LCPlight while converging/diverging it (depending on the non-zero opticalpower). When LCP light is incident on optical element 515, opticalelement 515 is configured to transmit the LCP light without change inpolarization or direction. In this example, optical element 515 has anegative optical power for RCP light and is configured to convert RCPlight into LCP light while diverging it. In some embodiments, opticalelement 515 is a PSH lens, described below with respect to FIGS. 6E-6H.

Referring to FIG. 5J, optical element 515 receives RCP light 540-B andconverts RCP light 540-B into LCP light 544-C while diverging it.Referring to FIG. 5K, optical element 515 receives LCP light 542-B andtransmits LCP light 542-B as LCP light 542-C without changing itsdirection or polarization.

FIGS. 5L and 5M illustrate optical element 517 that has a non-zerooptical power for LCP light and zero optical power for RCP light, whichcan be used in place of optical element 512 or optical element 513 inoptical element pair 500 or 502, respectively. In some embodiments,optical element 517 is configured to act as a lens when LCP light isincident on optical element 517 by converting the LCP light into RCPlight while converging/diverging it (depending on the non-zero opticalpower). When RCP light is incident on optical element 517, opticalelement 517 is configured to transmit the RCP light without change inpolarization or direction. In this example, optical element 517 has apositive optical power for LCP light and is configured to convert LCPlight into RCP light while converging it. In some embodiments, opticalelement 517 is a PSH lens, described below with respect to FIGS. 6E-6H.

Referring to FIG. 5L, optical element 517 receives LCP light 542-B andconverts LCP light 542-B into RCP light 546-C while converging it.

Referring to FIG. 5M, optical element 517 receives RCP light 540-B andtransmits RCP light 540-B as RCP light 540-C without changing itsdirection or polarization.

In some embodiments, second optical elements 515 and/or 517 can beactive optical elements configurable via controller 516 to be in a thirdoptical element state (e.g., “Off” state), as shown in FIGS. 5J-5M, or afourth optical element sate (e.g., “On” state), as shown in FIG. 5N.

FIGS. 6A-6D are schematic diagrams illustrating Pancharatnam-berry phase(PBP) lens 600 in accordance with some embodiments. In some embodiments,the second optical element 412 of an optical stage 420 in varifocaloptical assembly 400, described above with respect to FIGS. 4 and 5A-5H,includes PBP lens 600. In some embodiments, PBP lens 600 is a liquidcrystal optical element that includes a layer of liquid crystals. Insome embodiments, PBP lens 600 includes a layer of other type ofsubstructures, e.g., nanopillars composed of high refraction indexmaterials. PBP lens 600 adds or removes optical power based in part onpolarization of incident light. For example, if RCP light is incident onPBP lens 600, PBP lens 600 acts as a positive lens (i.e., it causeslight to converge). And, if LCP light is incident on the PBP lens, thePBP lens acts as a negative lens (i.e., it causes light to diverge). Insome embodiments, PBP lenses also change the handedness of light to theorthogonal handedness (e.g., changing LCP to RCP or vice versa). PBPlenses are also wavelength selective. If the incident light is at thedesigned wavelength, LCP light is converted to RCP light, and viceversa. In contrast, if incident light has a wavelength that is outsidethe designed wavelength range, at least a portion of the light istransmitted without change in its polarization and without focusing orconverging. PBP lenses may have a large aperture size and can be madewith a very thin liquid crystal layer. Optical properties of the PBPlens (e.g., focusing power or diffracting power) are based on variationof azimuthal angles (θ) of liquid crystal molecules. For example, for aPBP lens, azimuthal angle 0 of a liquid crystal molecule is determinedbased on Equation (1):

$\begin{matrix}{\theta = {\left( {\frac{r^{2}}{f}*\frac{\pi}{\lambda}} \right)/2}} & (1)\end{matrix}$

where r denotes a radial distance between the liquid crystal moleculeand an optical center of the PBP lens, f denotes a focal distance, and Adenotes a wavelength of light that the PBP lens is designed for. In someembodiments, the azimuthal angles of the liquid crystal molecules in thex-y plane increase from the optical center to an edge of the PBP lens.In some embodiments, as expressed by Equation (1), a rate of increase inazimuthal angles between neighboring liquid crystal molecules alsoincreases with the distance from the optical center of the PBP lens. ThePBP lens creates a respective lens profile based on the orientations(i.e., azimuthal angle θ) of a liquid crystal molecule in the x-y plane.In contrast, a (non- PBP) liquid crystal lens creates a lens profile viaa birefringence property (with liquid crystal molecules oriented out ofx-y plane, e.g., a non-zero tilt angle from the x-y plane) and athickness of a liquid crystal layer.

FIG. 6A illustrates a three-dimensional view of PBP lens 600 withincoming light 604 entering the lens along the z-axis.

FIG. 6B illustrates an x-y-plane view of PBP lens 600 with a pluralityof liquid crystals (e.g., liquid crystals 602-1 and 602-2) with variousorientations. The orientations (i.e., azimuthal angles 0) of the liquidcrystals vary along reference line between A and A′ from the center ofPBP lens 600 toward the periphery of PBP lens 600.

FIG. 6C illustrates an x-z-cross-sectional view of PBP lens 600. Asshown in FIG. 6C, the orientations of the liquid crystal (e.g., liquidcrystals 602-1 and 602-2) remain constant along z-direction. FIG. 6Cillustrates an example of a PBP structure that has constant variationalong z and birefringent thickness (Δn×t) that is ideally half of thedesigned wavelength, where Δn is the birefringence of the liquid crystalmaterial and t is the physical thickness of the plate. A PBP opticalelement (e.g., lens, grating) may have a liquid crystal structure thatis different from the one shown in FIG. 6C. For example, a PBP opticalelement may include a double twist liquid crystal structure along thez-direction. In another example, a PBP optical element may include athree-layer alternate structure along the z-direction in order toprovide achromatic response across a wide spectral range. FIG. 6Dillustrates a detailed plane view of the liquid crystals along thereference line between A and A′ shown in FIG. 6B. Pitch 606 is definedas a distance along the x-axis at which the azimuthal angle θ of aliquid crystal has rotated 180 degrees. In some embodiments, pitch 606varies as a function of distance from the center of PBP lens 600. In acase of a lens, the azimuthal angle 0 of liquid crystals varies inaccordance with Equation (1) shown above. In such cases, the pitch atthe center of the lens is longest and the pitch at the edge of the lensis shortest.

FIGS. 6E-6H are schematic diagrams illustrating a polarization sensitivehologram (PSH) lens in accordance with some embodiments. In someembodiments, the second optical element 412 of an optical element pairin varifocal optical assembly 400, described above with respect to FIGS.4 and 5A-5H, includes (PSH) lens 610. PSH lens 610 is a liquid crystalPSH lens including a layer of liquid crystals arranged in helicalstructures (e.g., a liquid crystal formed of a cholesteric liquidcrystal). Similar to a PBP lens (described above with respect to FIGS.6A-6D), a PSH lens adds or removes optical power based in part onpolarization of an incident light. However, PSH lens is selective withrespect to circular polarization of light. When state (handedness) ofthe circularly polarized light is along a helical axis of a liquidcrystal, the PSH lens interacts with the circularly polarized light andthereby changes the direction of the light (e.g., refracts or diffractsthe light). Concurrently, while transmitting the light, the PSH lensalso changes the polarization of the light. In contrast, the PSH lenstransmits light with opposite circular polarization without changing itsdirection or polarization. For example, a PSH lens changes polarizationof RCP light to LCP light and simultaneously focuses or defocuses thelight while transmitting LCP light without changing its polarization ordirection. Optical properties of the PSH lens (e.g., focusing power ofdiffracting power) are based on variation of azimuthal angles of liquidcrystal molecules. In addition, the optical properties of the PSH arebased on a helical axis and/or a helical pitch of a liquid crystal.

FIG. 6E illustrates a three-dimensional view of PSH lens 610 withincoming light 614 entering the lens along the z-axis. FIG. 6Eillustrates an x-y plane view of PSH lens 610 with a plurality of liquidcrystals (e.g., liquid crystals 612-1 and 612-2) with variousorientations. The orientations (i.e., azimuthal angle θ) of the liquidcrystals vary along reference line between B and B′ from the center ofPSH lens 610 toward the periphery of PSH lens 610. FIG. 6G illustratesan x-z-cross-sectional view of PSH lens 610. As shown in FIG. 6G, incontrast to PBP described with respect to FIG. 6C, the liquid crystals(e.g., liquid crystals 612-1 and 612-2 in FIG. 6F) of PSH lens 610 arearranged in helical structures 618. Helical structures 618 have helicalaxes aligned corresponding to the z-axis. As the azimuthal angle ofrespective liquid crystals on the x-y-plane varies, the helicalstructures create a volume grating with a plurality of diffractionplanes (e.g., planes 620-1 and 620-2) forming cycloidal patterns. Thediffraction planes (e.g., Bragg diffraction planes) defined in a volumeof an PSH lens produce a periodically changing refractive index. Helicalstructures 618 define the polarization selectivity of PSH lens 610, aslight with circular polarization handedness corresponding to the helicalaxis is diffracted while light with circular polarization with theopposite handedness is not diffracted. Helical structures 618 alsodefine the wavelength selectivity of PSH lens 610, as helical pitch 622determines which wavelength(s) are diffracted by PSH lens 610 (lightwith other wavelengths is not diffracted). For example, for a PSH lens,the designed wavelength for which the PSH lens will diffract the lightis determined based on Equation (2):

λ=2n_(eff)P_(z)   (2)

where λ denotes a wavelength of light that the PSH lens is designed for,P_(z) is distance of helical pitch 622, and n_(eff) is the effectiverefractive index of the liquid crystal medium that is a birefringentmedium. A helical pitch refers to a distance when a helix has made a 180degree turn along a helical axis (e.g., the z-axis in FIG. 6G). Theeffective refractive index of the birefringent liquid crystal medium isdetermined based on Equation (3):

$\begin{matrix}{n_{eff} = \sqrt{\frac{n_{0}^{2} + {2n_{e}^{2}}}{3}}} & (3)\end{matrix}$

where n₀ is the ordinary refractive index of the birefringent medium andn_(e) is the extraordinary refractive index of the birefringent medium.

FIG. 6H illustrates a detailed plane view of the liquid crystals alongthe reference line between B and B′ in FIG. 6F. Pitch 406 is defined asa distance along x-axis at which the azimuth angle of liquid crystal hasrotated 180 degrees from the initial orientation. In some embodiments,pitch 616 varies as a function of distance from the center of PSH lens610. In a case of a lens, the azimuthal angle of liquid crystals variesin accordance with Equation (1) shown above. In such cases, the pitch atthe center of the lens is the longest and the pitch at the edge of thelens is the shortest.

FIG. 7A illustrates optical paths of light transmitted through varifocaloptical assembly 700, corresponding to varifocal optical assembly 400,in accordance with some embodiments. As shown, in this example,varifocal optical assembly 700 includes a three-stage optical stackhaving optical stages 420-1, 420-2, and 420-3.

The first optical stage 420-1 is configured to receive first light 721having a first divergence. The first light is transmitted through theoptical stack and output from the last optical stage 420-3 as secondlight 727 having a second divergence that is different from the firstdivergence. In some embodiments, the second divergence is less than thefirst divergence (e.g., the second light is more collimated than thefirst light)

In some embodiments, the first optical stage 420-1 is configured toreceive first light 721. In this example, first optical element 410-1 offirst optical stage 420-1 is in the first state. Thus, first opticalelement 410-1 receives first light 721 having left-circular polarization(LCP) and converts the LCP first light 721 as light 722 havingright-circular polarization (RCP). Second optical element 412-1 of firstoptical stage 420-1 receives RCP light 722 having the first divergenceand converts RCP light 722 into third light 723 having left-circularpolarization (LCP) while focusing it, resulting in third light 723having a third divergence that is smaller than the first divergence(e.g., second optical element 412-1 acts as a converging lens and thusRCP light 722 is converted into more converged LCP third light 723).

A first optical element 410-2 of a second optical stage 420-2 is in thesecond state. Thus, second optical element 410-2 receives LCP thirdlight 723, output from first optical stage 420-1, and transmits LCPthird light 723 as LCP light 724 without changing the polarization.Second optical element 412-2 of second optical stage 420-2 receives LCPlight 724 having the third divergence and converts LCP light 724 intoRCP fourth light 725 while diverging it, resulting in fourth light 725having a fourth divergence that is larger than the third divergence(e.g., second optical element 412-2 acts as a diverging lens and thuslight 724 is converted into more diverging RCP fourth light 725).

A first optical element 410-3 of a third and last optical stage 420-3 isin the first state. Thus, first optical element 410-3 receives RCPfourth light 725, output from second optical stage 420-2, and convertsRCP fourth light 725 as LCP light 726. Second optical element 412-3receives LCP light 726 having the fourth divergence and converts LCPlight 726 into RCP fifth light 727 while diverging it, resulting infifth light 727 having a fifth divergence that is smaller than thefourth divergence (e.g., second optical element 412-2 acts as aconverging lens and thus light 726 is converted into more converged RCPfifth light 727). Since the third optical stage is the last opticalstage in the optical stack, the fifth light 727 having the fifthdivergence corresponds to (e.g., is the same as) the second light 727having the second divergence, output from the output end of the opticalstack as described above.

Thus, the additional optical stage 420-2 is configured to receive lightfrom a previous optical stage 420-1 and transmit the light to a nextoptical stage such that the light output from the additional opticalstage has a divergence that is different from the divergence of thelight received by the optical stage. The divergence of the transmittedlight is determined based on the divergence of the received light, thepolarization of the received light, and a state of the first opticalelement of the additional optical stage.

In some embodiments, varifocal optical assembly 700 may also include afirst polarizer 714 at the input side of the optical stack. In someembodiments, varifocal optical assembly 700 also includes, at the outputside of the optical stack, a switchable retarder 716 that iselectrically coupled to a controller 717 and a second polarizer 718.Switchable retarder 716 has optical properties that are similar to (orthe same as) those of first optical element 410 described above withreference to FIGS. 4 and 5A-5H, which are therefore not repeated herefor brevity. In some embodiments, switchable retarder 716 is aswitchable half wave plate. As explained in FIGS. 5A-5H, operation ofeach optical stage of varifocal optical assembly 700 is dependent on thepolarization of light incident on the optical stage. Although not shown,the optical elements in varifocal optical assembly 700 may not have 100%efficiency and thus, leakage at an optical element in varifocal opticalassembly 700 may contribute to “ghosting” effects. For example, opticalelements in varifocal optical assembly 700 may not exhibit perfectbirefringence across a spectral range, leading to less than 100%efficiency in converting polarization. Thus, first polarizer 714 andsecond polarizer 718, either individually or in combination, can be usedto block at least a portion of the leaked light, thereby reducing“ghosting” effects. First polarizer 714 is configured to ensure thatlight having only one polarization is incident upon the first opticalstage 420-1 and second polarizer 718 is configured to ensure that lighthaving only one polarization is output from varifocal optical assembly400. As shown in FIGS. 7B and 7C, light output from the last opticalstage 420-n may have a different polarization depending on the state ofrespective first optical elements 410 of respective optical stages 420.Thus, switchable retarder 716 is configured to transmit light outputfrom the last optical stage 420-n such that the output light has apolarization that corresponds to (e.g., can be transmitted through) thesecond polarizer 718. Thus, one or more of first polarizer 714 andsecond polarizer 718 may be included in varifocal optical assembly 700in order to reduce transmission of the leaked light having an undesiredpolarization. The leaked light corresponding to optical aberrations ordistortions that may be caused by transmitting light through multipleoptical stages that may have non-constant birefringence that across aspectral range of the transmitted light. Thus, depending on theuniformity of the birefringence across the spectral range of transmittedlight, one or more of polarizer 714 and polarizer 718 with switchablewaveplate 716 may be optional.

In some embodiments, such as when varifocal optical assembly 700includes first polarizer 714, light 720 having the first divergence maybe incident on first polarizer 714 and a portion of light 720 havingundesired polarization (in this example RCP light is the undesiredpolarization) may be transmitted through first polarizer 714, forexample due to high incident angle, as first light 721. In someembodiments, light having the undesired polarization can propagatethrough the optical stack. In some embodiments, the percentage ofundesirable light that may be transmitted through the system may be ashigh as 1%. In some embodiments, the percentage of undesirable lightthat may be transmitted through the system may be larger than 1%. Whenlight having the undesired polarization propagates through the opticalstack, the light having the undesired polarization will have one or moreoptical paths and polarization evolutions that are different from thelight path of light having the desired polarization. The various opticalpaths and polarization evolutions of the light having the undesiredpolarization is due to interaction of the light having the undesiredpolarization with the optical elements (e.g. first optical elements 410,second optical elements 412) of the optical stack. For example, theoptical elements of the optical stack may have an efficiency that isless than 100% or may have a non-constant response to light of differentwavelengths within a spectral range of the incident light or may have anon-constant response to incident light that are incident on the opticalelement at large angles. Thus, each time light interacts with a firstoptical element of the optical stack, a portion of the light (e.g., aportion having wavelengths towards the edges of the spectral range of aportion having large incident angles) may experience an undesiredretardance (e.g., any retardance other than the designed/desiredretardance) and the polarization of the portion of the light degradesfrom a circular polarization (e.g., LCP or RCP) to an ellipticalpolarization. It is undesirable to transmit light having an ellipticalpolarization, referred to hereafter as “elliptical light,” through theoptical stack since the ellipticity of the light will increase as theelliptical light is transmitted through each respective first opticalelement 410 of the successive optical stages 420 of the optical stack.Additionally, elliptical light incident on a second optical element 412of the optical stack will be split between different diffractive ordersresulting in “leaked light” (e.g., a first portion of the ellipticallight may be diffracted to a desired 1^(st) diffraction order, a secondportion of the elliptical light may be diffracted to an undesired 0^(th)diffraction order, and a third portion of the elliptical light may bediffracted to an undesired −1^(st) diffraction order). Both of thephenomena described above, light accumulating an undesired retardanceresulting in an elliptical polarization and light being diffracted intoan undesired diffraction order, may be cumulative as the lightpropagates through the optical stack resulting in “ghosting effects”with multiple “ghost paths.” A first portion of light that contributesto the “ghost paths” may have a polarization that is the same as thedesired output polarization and will be transmitted to the user's eyealong with the desired light. However, a second portion of the lightthat contributes to the “ghost paths” may have a polarization that isthe different from (e.g., orthogonal to) the desired output polarizationand can be blocked (e.g., eliminated) from reaching the user's eyes byone or more of first polarizer 716 and second polarizer 718.

For example, when varifocal optical assembly 700 includes switchableretarder 716 and controller 717, switchable retarder 716 receives light727 that may include light having the desired polarization (for example,RCP light), referred hereafter as the “desired light,” as well as lighthaving the undesired polarization (for example, LCP light), referredhereafter as the “undesired light.” In some cases, the desired light hasa higher intensity than undesired light. In this example, switchableretarder 716 is in the first state and thus converts the undesired LCPlight into undesired RCP light and converts desired RCP light intodesired LCP light. Thus, light 728 output from switchable retarder 716includes both desired LCP light and undesired RCP light. Secondpolarizer 718 is configured to receive desired LCP light and transmitthe desired LCP light as light 729. Second polarizer 718 is alsoconfigured to receive undesired RCP light and to absorb the undesiredRCP light, thus reducing the number of “ghost paths” that contribute to“ghosting effects.”

FIGS. 7B-7C show examples of different settings of a varifocal opticalassembly in accordance with some embodiments. FIGS. 7B and 7C show theoptical power that light transmitted through an optical stack willacquire at each optical stage, as well as the state of a respectivefirst optical element 410 of each optical stage. FIG. 7B shows theprogression of RCP light incident on the optical stack and FIG. 7C showsthe progression of LCP light incident on the optical stack. As shown,different resulting optical powers (see columns 770 and 790) can beachieved by adjusting the state of respective first optical elements 410in the optical stack.

Although specific numerical values are provided here as an example, oneof ordinary skill in art can understand that different optical powerscan be used and achieved without changing the principles and methodsdescribed herein.

FIGS. 8A-8B illustrate a display device 800 that includes varifocaloptical assembly 400 in accordance with some embodiments. As shown inFIG. 8A, display device 800 includes display 810, corresponding todisplay 310 and display 360, and varifocal optical assembly 400. Detailsregarding varifocal optical assembly 400 that are provided above withrespect to FIG. 4 are not repeated here for brevity. Display 810 isconfigured to emit image light corresponding to one or more imagestoward varifocal optical assembly 400. Varifocal optical assembly 400 isconfigured to receive the image light emitted from display 810 andtransmit the image light toward an eyebox 780 or a pupil 350 of a user'seye 340.

In some embodiments, display device 800 is a head-mounted displaydevice. In near-eye display devices and head-mounted display devices,varifocal optical assembly 400 is located between the eyes 340 of theuser and the display in order to allow the user to comfortably viewimages displayed by image source 410 even if the image source 410 islocated outside the accommodation range of the eyes of the user.

In some embodiments, display device 800 may also include a lens assembly812, corresponding to lens assembly 362. Lens assembly 812 may include,for example, one or more of a conventional lens, a pancake lens, a PSHoptical lens, a geometric phase lens, a PBP lens, and any other opticalelement that has lensing (e.g., focusing) properties.

In some embodiments, as shown in FIG. 8A, lens assembly 812 may belocated between display 810 and varifocal optical assembly 400. In suchcases, lens assembly 812 is configured to receive image light outputfrom display 810 and transmit the image light toward varifocal opticalassembly 400. Alternatively, as shown in FIG. 8B, varifocal opticalassembly 400 may be located between display 810 and lens assembly 812.In such cases, lens assembly 812 is configured to receive light outputfrom varifocal optical assembly 400 and transmit the light toward auser's eye 340 or an eyebox 780. Lens assembly 812 may also beconfigured to reduce optical aberrations (e.g., distortion, pupil swim,etc).

In some embodiments, varifocal optical assembly 400 may also include anoptically transparent substrate 820 that is configured to add rigidityto the optical stack. Although FIG. 8A shows optically transparentsubstrate 820 located at the output end of the optical stack, betweenthe second polarizer 718 and a user's eye 340, optically transparentsubstrate 820 can be located anywhere varifocal optical assembly 400,including in between or adjacent to one or more optical stages withoutdetriment or degradation to the function of the varifocal opticalassembly 400. In some embodiments, one or more optically transparentsubstrates 820 may be located at one or more of the input end and theoutput end of the optical stack. In some embodiments, opticallytransparent substrate 820 is included in varifocal optical assembly 400in order to reduce pupil swim. In some embodiments, pupil swim may becaused by non-rigid (e.g., wavy) film elements in the optical stack.Thus, the addition of optically transparent substrate 820 to the opticalstack may increase the rigidity of the optical stack, thereby reducingpupil swim.

In some embodiments, as shown in FIG. 8B, display device 800 may belocated (e.g., encased, enclosed) in a housing or frame 830 such thatdisplay device 800 is configured to be mounted near a user's eyes 340and to operate as a head-mounted display device.

FIG. 9 illustrates a method 900 of adjusting the focal length of lighttransmitted through a varifocal optical assembly in accordance with someembodiments.

Method 900 includes (step 910) transmitting light through an opticalstack that has a plurality of stages (e.g., successive optical stages420-1, 420-2, 420-m, 420-m). Each stage of the plurality of opticalstages is configurable to be in any of a plurality of states including afirst state and a second state. In the first state, the respectiveoptical stage has a first respective optical power for light of a firstpolarization and a second respective optical power, different from thefirst respective optical power, for light of a second polarization thatis orthogonal to the first polarization. In the second state, therespective optical stage has a third optical power for light of thefirst polarization and a fourth optical power for light of the secondpolarization.

Method 900 also includes (step 920) adjusting a focal length of theoptical stack by changing respective states of one or more opticalstages of the plurality of optical stages.

In light of these principles, we now turn to certain embodiments of avarifocal optical assembly, a display device including the varifocaloptical assembly, and a method of transmitting light through a varifocaloptical assembly.

In accordance with some embodiments, an optical assembly (e.g.,varifocal optical assembly 400) includes a plurality of optical elementsthat are configured to transmit light in successive optical stages(e.g., optical stages 420-1, 420-2, 420-n). Each respective opticalstage of the successive optical stages includes at least one respectiveoptical element of the plurality of optical elements and is configurableto be in any of a plurality of states including a first state and asecond state. In the first state (e.g., “off” state), the respectiveoptical stage has a first respective optical power for light of a firstpolarization and a second respective optical power, different from(e.g., orthogonal to) the first respective optical power, for light of asecond polarization that is orthogonal to the first polarization. In thesecond state (e.g., “on” state), the respective optical stage has athird optical power for light of the first polarization and a fourthoptical power for light of the second polarization. An overall opticalpower of the optical assembly is variable by configuring one or more ofthe successive optical stages.

In some embodiments, an optical assembly (e.g., varifocal opticalassembly 400) includes a plurality of optical elements of a first type(e.g., first optical element 410) and a plurality of optical elements ofa second type (e.g., second optical element 412). The optical element ofthe first type is configurable to be in a first state (e.g., “off”state) or a second state (e.g., “on” state). In the first state, therespective optical element of the first type converts light of a firstor second polarization into light of a second or first polarization,respectively (e.g., converting LCP light to RCP light and vice versa).The first polarization is orthogonal to the second polarization. In thesecond state, the respective optical element of the first type transmitsincident light without changing its polarization. The respective opticalelement of the second type is configured to receive light transmittedthrough the respective optical element of the first type. The respectiveoptical element of the second type has an optical power that isdependent on whether the light transmitted through the optical elementof the first type has the first polarization or the second polarization.

In some embodiments, the second respective optical power is equal (e.g.,in magnitude) to and opposite (e.g., in sign) from the first respectiveoptical power. In some embodiments, the second respective optical poweris less than (e.g., in magnitude) the first respective optical power. Insome embodiments, the second respective optical power is zero.

In some embodiments, the successive optical stages (e.g., optical stages420-1, 420-2, 420-m, 420-n) form an optical stack that is configurableto receive input light (e.g., first light 721) having a first divergenceand to project output light (e.g., second light 727) having a seconddivergence distinct from the first divergence. The second divergence isadjustable by changing the state of at least one of the successiveoptical stages.

In some embodiments, the second divergence is less than the firstdivergence.

In some embodiments, the optical stack is configurable to receive inputlight (e.g., first light 721) on an input side and to project outputlight (e.g., second light 727) on an output side. The successive opticalstages include a first optical stage (e.g., first optical stage 420-1)at the input side. The first optical stage has a fifth optical powerdepending on the state of the first stage and a polarization of theinput light.

In some embodiments, the plurality of successive optical stages includesone or more additional optical stages (e.g., optical stages 420-2,420-m). Each respective optical stage of the one or more additionaloptical stages has a respective optical power depending on the state ofthe respective optical stage and the polarization of light output froman optical stage preceding the respective optical stage.

In some embodiments, light output from an optical stage preceding arespective optical stage has a third divergence. The respective opticalstage is configurable to output light having a fourth divergence. Thefourth divergence is determined based on the third divergence, the stateof the respective optical stage, and the polarization of the lightoutput from the optical stage preceding the respective optical stage.

In some embodiments, the optical assembly (e.g., varifocal opticalassembly 400) further includes an optically transparent substrate (e.g.,optically transparent substrate 820) configured to add rigidity to theoptical stack.

In some embodiments, the optically transparent substrate (e.g.,optically transparent substrate 820) can be located at any location inthe optical assembly. In some embodiments, the optically transparentsubstrate may be located at one or more of the input end and the outputend of the optical stack. In some embodiments, the optically transparentsubstrate is included in the optical stack in order to reduce pupilswim.

In some embodiments, the optical stack has an input side and an outputside. The successive optical stages (e.g., optical stages 420-1, 420-2,420-m, 420-n) includes a first stage (e.g., first stage 420-1) on theinput side and a second stage (e.g., last stage 420-n) on the secondside. The first stage is configurable to have a first optical power(e.g., a first optical power magnitude) and the second stage isconfigurable to have a second optical power (e.g., a second opticalpower magnitude). The optical stack is configured such that the firststage and the second stage have different optical powers (e.g.,different optical power magnitudes).

In some embodiments, the optical stack is configured such that thesecond stage has a greater optical power (e.g., has a greater opticalpower magnitude) than the first stage. In some embodiments, the secondoptical power is larger than (e.g., has a magnitude that is larger than)the first optical power.

In some embodiments, the optical stack has an input side and an outputside. The optical assembly (e.g., varifocal optical assembly 400)further includes a first polarizer (e.g., first polarizer 714) on theinput side of the optical stack.

In some embodiments, the optical assembly (e.g., varifocal opticalassembly 400) further includes a second polarizer (e.g., secondpolarizer 718) and a switchable retarder (e.g., switchable retarder716). In some embodiments, the switchable retarder is disposed betweenthe second polarizer and the output side of the optical stack.

In some embodiments, an optical element of the second type includes athin film formed on a surface of the optical element of the first type(e.g., the second optical element 412 includes a thin film formed asurface of the first optical element 410).

In some embodiments, the optical element of the second type (e.g.,second optical element 412) includes one or more of aPancharatnam-berry-phase (PBP) lens, a polarization sensitive hologram(PSH) lens, and a liquid crystal optical phase array. In someembodiments, the optical element of the second type includes one or moreof a PBP grating and a PSH grating. In some embodiments, the opticalelement of the second type includes a geometric-phase lens or ageometric-phase grating.

In some embodiments, one or more optical stages of the successiveoptical stages includes an active optical element (e.g., active secondoptical element 412). The active optical element is configurable to bein any of an “off” state and an “on” state. In the “off” state, theactive optical element has an optical power that is dependent on whetherthe light incident on the active optical element has the firstpolarization or the second polarization. In the “on” state, the activeoptical element transmits light incident on the active optical elementwithout changing polarization or direction of the incident lightregardless of the polarization of the incident light.

In some embodiments, a magnitude of the optical power of any of thefirst optical power, the second optical power, the third optical powerand the fourth optical power at any of the successive optical stages isno greater than 2.0 diopters. For example, any of the first opticalpower, the second optical power, the third optical power and the fourthoptical power at any of the successive optical stages is between andincludes −2.0 diopters and +2.0 diopters. In some embodiments, amagnitude of the optical power of a stage of the successive stages is nogreater than 2.0 diopters. For example, an optical power of a stage ofthe successive stages is between and includes −2.0 diopters and +2.0diopters.

In some embodiments, the overall optical power of the optical assemblyis variable to be at any of at least 3 different levels. In someembodiments, when the optical assembly includes at least two stages,each stage having a different optical power, the overall optical powerof the optical assembly is variable to be at any of at least 4 differentlevels. In some embodiments, when the optical assembly includes at leasttwo stages, each stage having a different optical power and including anactive second optical element (e.g., active PBP, active PVH, activeliquid crystal optical phase array), the overall optical power of theoptical assembly is variable to be at any of at least 5 differentlevels.

In accordance with some embodiments, a display device (e.g., displaydevice 800) includes a display (e.g., display 810) configured to emitimage light and an optical assembly (e.g., varifocal optical assembly400) configured to transmit image light. The optical assembly includes aplurality of optical elements that are configured to transmit light insuccessive optical stages (e.g., optical stages 420-1, 420-2, 420-n).Each respective optical stage of the successive optical stages includesat least one respective optical element of the plurality of opticalelements and is configurable to be in any of a plurality of statesincluding a first state and a second state. In the first state (e.g.,“off” state), the respective optical stage has a first respectiveoptical power for light of a first polarization and a second respectiveoptical power, different from (e.g., orthogonal to) the first respectiveoptical power, for light of a second polarization that is orthogonal tothe first polarization. In the second state (e.g., “on” state), therespective optical stage has a third optical power for light of thefirst polarization and a fourth optical power for light of the secondpolarization. An overall optical power of the optical assembly isvariable by configuring one or more of the successive optical stages.

In some embodiments, the one or more optical stages of the successiveoptical stages (e.g., optical stages 420-1, 420-2, 420-m, 420-n)includes an optical element of a first type (e.g., first optical element410) and an optical element of a second type (e.g., second opticalelement 412). The optical element of the first type is configurable tobe in a first state (e.g., “off” state) or a second state (e.g., “on”state). In the first state, the respective optical element of the firsttype converts light of a first or second polarization into light of asecond or first polarization, respectively (e.g., converting LCP lightto RCP light and vice versa). The first polarization is orthogonal tothe second polarization. In the second state, the respective opticalelement of the first type transmits incident light without changing itspolarization. The respective optical element of the second type isconfigured to receive light transmitted through the respective opticalelement of the first type. The respective optical element of the secondtype has an optical power that is dependent on whether the lighttransmitted through the optical element of the first type has the firstpolarization or the second polarization.

In some embodiments, the successive optical stages (e.g., optical stages420-1, 420-2, 420-m, 420-n) form an optical stack configurable toreceive input light (e.g., light 721) having a first divergence and toproject output light (e.g., light 727) having a second divergence thatis distinct from the first divergence. The second divergence isadjustable by changing the state of at least one of the successiveoptical stages.

In some embodiments, the successive optical stages (e.g., optical stages420-1, 420-2, 420-m, 420-n) include a first optical stage (e.g., firstoptical stage 420-1) at an input side of the optical stack. The firststage has a fifth optical power depending on the state of the firststage and a polarization of input light (e.g., light 721). Thesuccessive optical stages also include one or more additional opticalstages (e.g., optical stages 420-2, 420-m). Each respective opticalstage of the one or more additional optical stages has a respectiveoptical power depending on the state of the respective optical stage andthe polarization of light output from an optical stage preceding therespective optical stage.

In some embodiments, the display device (e.g., display device 800) alsoincludes a lens assembly (e.g., lens assembly 812). In some embodiments,the lens assembly is located between the display (e.g., display 810) andthe optical assembly (e.g., varifocal optical assembly 400) and isconfigured to receive image light output from the display and transmitthe image light toward the optical assembly. In some embodiments, theoptical assembly is located between the display and the lens assembly.In some embodiments, the lens assembly is polarization selective lensassembly that has a fourth optical power for light having a firstpolarization and a fifth optical power, different from the fourthoptical power, for light having a polarization that is different from(e.g., orthogonal to) the first polarization. In some embodiments, thelens assembly is configured to transmit light having a specified (e.g.,either the first polarization or the second polarization) toward auser's eye with a non-zero optical power.

In some embodiments, the display device (e.g., display device 800) is ahead-mounted display device.

In accordance with some embodiments, a method (e.g., method 900) oftransmitting light includes (e.g., step 910) transmitting light throughan optical stack having a plurality of stages (e.g., successive opticalstages 420-1, 420-2, 420-m, 420-m). Each stage of the plurality ofoptical stages is configurable to be in any of a plurality of statesincluding a first state and a second state. In the first state, therespective optical stage has a first respective optical power for lightof a first polarization and a second respective optical power, differentfrom the first respective optical power, for light of a secondpolarization that is orthogonal to the first polarization. In the secondstate, the respective optical stage has a third optical power for lightof the first polarization and a fourth optical power for light of thesecond polarization.

Method 900 also includes adjusting a focal length of the optical stackby changing respective states of one or more optical stages of theplurality of optical stages (e.g., step 920).

Although various drawings illustrate operations of particular componentsor particular groups of components with respect to one eye, a personhaving ordinary skill in the art would understand that analogousoperations can be performed with respect to the other eye or both eyes.For brevity, such details are not repeated herein.

Although some of various drawings illustrate a number of logical stagesin a particular order, stages which are not order dependent may bereordered and other stages may be combined or broken out. While somereordering or other groupings are specifically mentioned, others will beapparent to those of ordinary skill in the art, so the ordering andgroupings presented herein are not an exhaustive list of alternatives.Moreover, it should be recognized that the stages could be implementedin hardware, firmware, software or any combination thereof.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the scope of the claims to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen in order to best explain theprinciples underlying the claims and their practical applications, tothereby enable others skilled in the art to best use the embodimentswith various modifications as are suited to the particular usescontemplated.

What is claimed is:
 1. An optical assembly, comprising: a plurality ofoptical elements configured to transmit light in successive opticalstages, each respective optical stage of the successive optical stagesincluding at least one respective optical element of the plurality ofoptical elements and is configurable to be in any of a plurality ofstates including a first state and a second state, wherein in the firststate, the respective optical stage has a first respective optical powerfor light of a first polarization and a second respective optical power,different from the first respective optical power, for light of a secondpolarization that is orthogonal to the first polarization; in the secondstate, the respective optical stage has a third optical power for lightof the first polarization and a fourth optical power for light of thesecond polarization; and an overall optical power of the opticalassembly is variable by configuring one or more of the successiveoptical stages.
 2. The optical assembly of claim 1, wherein one or moreoptical stages of the successive optical stages includes an opticalelement of a first type and an optical element of a second type,wherein: the optical element of the first type is configurable to be inan “off” state or an “on” state, wherein in the “off” state, the opticalelement of the first type converts light of the first or secondpolarization into light of the second or first polarization,respectively; and in the “on” state, the optical element of the firsttype transmits incident light without changing its polarization; theoptical element of the second type is configured to receive lighttransmitted through the optical element of the first type and has anoptical power that is dependent on whether the light transmitted throughthe optical element of the first type has the first polarization or thesecond polarization.
 3. The optical assembly of claim 1, wherein thesuccessive optical stages form an optical stack that is configurable toreceive input light having a first divergence and to project outputlight having a second divergence distinct from the first divergence, thesecond divergence being adjustable by changing the state of at least oneof the successive optical stages.
 4. The optical assembly of claim 3,wherein: the optical stack is configurable to receive input light on aninput side and to project output light on an output side; the successiveoptical stages include a first optical stage at the input side; and thefirst optical stage has a fifth optical power depending on the state ofthe first stage and a polarization of the input light.
 5. The opticalassembly of claim 4, wherein: the successive optical stages include oneor more additional optical stages; and each respective optical stage ofthe one or more additional optical stages has a respective optical powerdepending on the state of the respective stage and the polarization oflight output from a stage preceding the respective stage.
 6. The opticalassembly of claim 5, wherein: the light output from the optical stagepreceding the respective optical stage has a third divergence; therespective optical stage is configurable to output light having a fourthdivergence; and the fourth divergence is determined based on the thirddivergence, the state of the respective optical stage, and thepolarization of the light output from the optical stage preceding therespective optical stage.
 7. The optical assembly of claim 3, furthercomprising an optically transparent substrate configured to add rigidityto the optical stack.
 8. The optical assembly of claim 3, wherein: theoptical stack has an input side and an output side; and the successiveoptical stages include a first stage on the input side and a secondstage on the output side; the first stage is configurable to have afirst optical power; the second stage is configurable to have a secondoptical power; and the optical stack is configured such that the firststage and the second stage have different optical powers.
 9. The opticalassembly of claim 8, wherein the optical stack is configured such thatthe second stage has greater optical power than the first stage.
 10. Theoptical assembly of claim 3, wherein the optical stack has an input sideand an output side, the optical assembly further comprising a firstpolarizer on the input side of the optical stack.
 11. The opticalassembly of claim 10, further comprising: a second polarizer; and aswitchable retarder disposed between the second polarizer and the outputside of the optical stack.
 12. The optical assembly of claim 2, whereinthe optical element of the second type includes a thin film formed on asurface of the optical element of the first type.
 13. The opticalassembly of claim 2, wherein the optical element of the second typeincludes one or more of a Pancharatnam-berry-phase lens, a polarizationsensitive hologram lens, and a liquid crystal optical phase array. 14.The optical assembly of claim 1, wherein one or more optical stages ofthe successive optical stages includes an active optical element,wherein: the active optical element is configurable to be in any of an“off” state or an “on” state, wherein in the “off” state, the activeoptical element has an optical power that is dependent on whether lightincident on the active optical element of has the first polarization orthe second polarization; and in the “on” state, the active opticalelement transmits light incident on the active optical element withoutchanging polarization or direction of the incident light.
 15. Theoptical assembly of claim 1, wherein: a magnitude of any of the firstoptical power, the second optical power, the third optical power and thefourth optical power at any of the successive optical stages is nogreater than 2.0 diopters; and the overall optical power of the opticalassembly is variable to be at any of at least 3 different levels.
 16. Adisplay device, comprising: a display configured to emit image light;and an optical assembly configured to transmit image light, wherein theoptical assembly includes: a plurality of optical elements configured totransmit light in successive optical stages, each respective opticalstage of the successive optical stages including at least one respectiveoptical element of the plurality of optical elements and is configurableto be in any of a plurality of states including a first state and asecond state, and wherein in the first state, the respective opticalstage has a first respective optical power for light of a firstpolarization and a second respective optical power, different from thefirst respective optical power, for light of a second polarization thatis orthogonal to the first polarization; in the second state, therespective optical stage has a third optical power for light of thefirst polarization and a fourth optical power for light of the secondpolarization; and an overall optical power of the optical assembly isvariable by configuring one or more of the successive optical stages.17. The display device of claim 16, wherein one or more optical stagesof the successive optical stages includes an optical element of a firsttype and an optical element of a second type, wherein: the opticalelement of the first type is configurable to be in an “off” state or an“on” state, wherein in the “off” state, the optical element of the firsttype converts light of the first or second polarization into light ofthe second or first polarization, respectively; and in the “on” state,the optical element of the first type transmits incident light withoutchanging its polarization; the optical element of the second type isconfigured to receive light transmitted through the optical element ofthe first type and has an optical power that is dependent on whether thelight transmitted through the optical element of the first type has thefirst polarization or the second polarization.
 18. The display device ofclaim 16, wherein: the successive optical stages form an optical stackthat is configurable to receive input light having a first divergenceand to project output light having a second divergence distinct from thefirst divergence, the second divergence being adjustable by changing thestate of at least one of the successive optical stages.
 19. The displaydevice of claim 18, wherein: the successive optical stages include afirst optical stage at the input side of the optical stack and one ormore additional optical stages; the first optical stage has a fifthoptical power depending on the state of the first stage and apolarization of the input light; and each respective optical stage ofthe one or more additional optical stages has a respective optical powerdepending on the state of the respective optical stage and thepolarization of light output from an optical stage preceding therespective optical stage.
 20. A method of transmitting light, the methodcomprising: transmitting light through an optical stack having aplurality of optical stages, each stage of the plurality of opticalstages is configurable to be in any of a plurality of states including afirst state and a second state; adjusting a focal length of the opticalstack by changing respective states of one or more optical stages of theplurality of optical stages; wherein: in the first state, the respectiveoptical stage has a first respective optical power for light of a firstpolarization and a second respective optical power, different from thefirst respective optical power, for light of a second polarization thatis orthogonal to the first polarization; in the second state, therespective optical stage has a third optical power for light of thefirst polarization and a fourth optical power for light of the secondpolarization.